Proton Source Efficiency for Heterogeneous Distribution of Actinides in the Core of an Accelerator-Driven System

The distribution of actinides in the core of an accelerator-driven system loaded with plutonium, americium, and curium has been studied in order to optimize the proton source efficiency *. The optimization of * was performed by keeping some important characteristics of the system, e.g., the radial power profile and the reactivity of the core, constant. One of the basic assumptions of the study, that the magnitude of * is sensitive primarily to the composition of actinides in the inner part of the core, whereas only marginally to that in the outer part, has been confirmed. It has been shown that the odd-N nuclides (those nuclides with an even number of neutrons) in general and 241Am and 244Cm in particular have favorable properties with respect to improving * if they are placed in the innermost part of the core. The underlying reason for this phenomenon is that the energy spectrum of the source neutrons in the inner part of the core is harder than that of the average fission neutrons. Moreover, it has been shown that loading the inner part of the core with only curium increases * by ~7%. Plutonium, on the other hand, in particular high-quality plutonium consisting mainly of 239Pu and 241Pu, was found to be a comparatively source inefficient element and is preferably located in the outer part of the core. The differences in * are due to combined effects from relative changes in the average fission and capture cross sections and in the average fission neutron yield.